Fama Fraternitatis is the first of a series of spiritual manifestos published in 1614 in present-day Germany. It claimed to reveal to the world the existence of an age-old secret order — the Rosenkreutz — and championed pretty much every esoteric tradition imaginable in its name, from kabbalah to alchemy.
It is my understanding that these works were taken very seriously at the time, and even went on to shape some aspects of then burgeoning Freemasonry. Eventually, it broke into the mainstream with the foundation of a self-described Rosicrucian organization named AMORC during the 1910s — one that anybody can join.
That being said, no evidence for the existence of a Rosicrucian order prior to the publication of the aforementioned manifesto has ever been unearthed. It has however been suggested that some purported Rosicrucian writings are effectively a farce; Johannes Andreae admitted as much when he claimed authorship of the anonymous Chemycal Marriage of 1616 and described it as a practical joke.
Could it be that Rosicrucianism is originally nothing more than a humorous hoax? A bit of performance art of sorts which many in learned circles took at face value, and which subsequently developed a life of its own?
There is no direct evidence for the existence of the order at that time the manifestos were published. But as this was a secret society you very quickly run into a circle of logic where the adherents of the order would want to no evidence, so nothing conclusive can be stated.
The author of the manifestos did take a lot of care to print some fairly deep occult truths within the document. The first manifesto " Fama Fraternitatis " contains many aspects of advanced occult gematria that would be very rare knowledge to have in the time period. This would narrow the list of potential authors to someone who has made a dedicated study of the principles involved. It would also suggest that the author at least believed in the principles conveyed.
The third manifesto. "Chymical Wedding of Christian Rosenkreutz" contains a lot of symbolism drawing it's meanings from different religious and occult backgrounds, including Christian and eastern traditions. The author of this manifesto had a great understanding of these symbols.
It would be fair to say that regardless of the intentions of the author the profound impact that the manifestos have had, even today; would make it either one of the most successful hoaxes of all time. Or a very successful attempt to expand an order of occult practitioners.